The present invention relates to standardized mechanical interface systems for reducing particle contamination and more particularly to apparatus employing sealed containers suitable for use in semiconductor processing equipment to prevent particle contamination.
A standardized mechanical interface (SMIF) has been proposed to reduce particle contamination by significantly reducing particle fluxes onto wafers. This end is accomplished by mechanically ensuring that during transport, storage and processing of the wafers, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient outside environment do not enter the immediate internal wafer environment.
Control of particulate contamination is imperative for cost effective, high-yielding and profitable manufacturing of VLSI circuits. Because design rules increasingly call for smaller and smaller lines and spaces, it is necessary to exert greater and greater control on the number of particles and to remove particles with smaller and smaller diameters.
Some contamination particles cause incomplete etching in spaces between lines, thus leading to an unwanted electrical bridge. In addition to such physical defects, other contamination particles may cause electrical failure due to induced ionization or trapping centers in gate dielectrics or junctions.
The main sources of particulate contamination are personnel, equipment, and chemicals. Particles given off by personnel are transmitted through the environment and through physical contact or migration onto the wafer surface. People, by shedding of skin flakes, for example, are a significant source of particles that are easily ionized and cause defects. Although clean room garments reduce particle emissions they do not fully contain the emissions. It has been found that as many as 6000 particles per minute are emitted into an adjacent one cubic foot of space by a fully suited operator.
To control contamination particles, the trend in the industry is to build more elaborate (and expensive) clean rooms with HEPA and ULPA recirculating air systems. Filter efficiencies of 99.999% and up to ten complete air exchanges per minute are required to obtain an acceptable level of cleanliness.
Particles within the equipment and chemicals are termed "process defects." To minimize process defects, processing equipment manufacturers must prevent machine generated particles from reaching the wafers, and suppliers of gases and liquid chemicals must delivers cleaner products. Most important, a system must be designed that will effectively isolate wafers from particles during storage, transport and transfer into processing equipment. The Standard Mechanical Interface (SMIF) system has been proposed to achieve this goal.
The SMIF concept is based on the realization that a small volume of still, particle-free air, with no internal source of particles, is the cleanest possible environment for wafers. Further details of one proposed system are described in the article "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING", by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115 and in the above cross-referenced application.
The proposed SMIF system has three main components, namely, (1) minimum volume, dustproof boxes are used for storing and transporting wafer cassettes; (2) canopies are placed over cassette ports of processing equipment so that the environments inside the boxes and canopies become miniature clean spaces; (3) doors on the boxes are designed to mate with doors on the interface ports on the equipment canopies and the two doors are opened simultaneously so that particles which may have been on the external door surfaces are trapped ("sandwiched") between the doors.
In the proposed SMIF system, a box is placed at the interface port on top of the canopy; latches release the box door and the canpoy port door simultaneously. A mechanical elevator lowers the two doors, with the cassette riding on top, into the canopy covered space. A manipulator picks up the cassette and places it onto the cassette port/elevator of the equipment. After processing, the reverse operation takes place.
The SMIF system has been proved effective by experiments using prototype SMIF components both inside and outside a clean room. The SMIF configuration achieved a ten-fold improvement over the conventional handling of open cassettes inside the clean room.
Modern processing equipment must be concerned with particle sizes which range from below 0.01 micrometers to above 200 micrometers. Particles with these sizes can be very damaging in semiconductor processing. Typical semiconductor processes today employ geometrics which are 1 micrometer and under. Unwanted contamination particles which have geometries measuring greater than 0.1 micrometer substantially interfere with 1 micrometer geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries.
In typical processing environments today, "clean rooms" are established in which, through filtering and other techniques, attempts are made to remove particles having geometries of 0.03 micrometer and above. There is a need, however, to improve the processing environment. The conventional "clean room" cannot be maintained as particle free as desired. It is virtually impossible to maintain conventional clean rooms free of particles of a 0.01 micrometer size and below.
For this reason, systems such as the SMIF system have come under consideration. The proposed SMIF systems, however, have not been fully satisfactory. The SMIF systems which have been proposed have not permitted a full seal to be maintained between the interior of the box and the interior of the canopy during a cassette transfer. The mechanisms for latching and unlatching both the doors have not been convenient and adequate for making the cassette transfer. Accordingly, there is a need for improved mechanisms.
The proposed SMIF system, however, has some deficiencies. When SMIF boxes become contaminated, it is very difficult to remove small contaminant particles since the force of attraction of small particles to surfaces of equipment is very high. When small particles become attached to a surface such as a SMIF box, they are not effectively removed by filtration techniques. Circulating and filtering air or other gas within a box does not readily remove the contamination particles which are attracted and held in contact with surfaces.
Whenever an object such as a SMIF box is disturbed, by bumping for example, many small particles are freed from the surface and find their way as contaminants into any semiconductor or other article which is present in the box. Also, moving parts of a box system tend to create contamination particles whenever surfaces rub together. Therefore, it is desirable to avoid movement of articles such as wafers, within a box and to otherwise avoid the rubbing of parts.
While scrubbing and washing techniques have been developed for removing small particles from equipment and surfaces, these processes tend to be cumbersome and furthermore are not entirely effective. Where possible, it is desirable to avoid generation of contamination particles rather than rely upon their removable.
While proposed SMIF systems have been effective for excluding many unwanted contaminants, they have not been fully effective and need to be improved.